Re: [ESPResSo-users] Triple Cos term formula for Dihedral potential

[Muhammad Anwar]

Axel Arnold wrote:
> Hi,
>
> the better option is to copy the relevant code for the dihedral interaction, 
> and make a second one with a different name, e.g. dihedral3cos. As for the 
> implementation, best simply copy the dihedral.h and replace there the force 
> and energy formulas, and of course the names. You then just need to modify 
> interaction_data.[ch], energy.h, force.h and pressure.h to add  the glue code. 
> You can basically just copy the dihedral code, and have to find a constant in 
> interaction_data.h (BONDED_IA_DIHEDRAL3COS).
>
> Many regards,
> Axel
>
> On Thursday 26 May 2011 08:47:19 Muhammad Anwar wrote:
>   
> > Hello,
> > Dear Users,
> > I want to use triple cos term dihedral potential in my simulation, which is
> > V = 0.5 *( K1 * (1 - cos(phi) + K2 * (1 - cos(2*phi) + K3 * (1 -
> > cos(3phi) ).. Can anybody give any suggestion, how can i amend the given
> > dihedral potential to use this potential? There may be two
> > possibilities, one in which i just put the tcl command like this
> >
> > inter 3 dihedral 1 1 0
> > and change the dihedral file according to my need by putting the
> > different values of K and phi, but this would be more specific for only
> > one case. The second option may be i write separate header file for it
> > and use it. If i use first option i will just change the fac value by
> > adding new terms for force and energy. Is it ok? or should i write new
> > code for it, if yes how can i write and add? Please guide me?
> > Thanks
> >
> > Best Regards,
> > Muhammad Anwar
> >
> >
> >     

Hi,
Dear Dr Arnold,
I am modifying the dihedral potential code, I am not understanding how 
and why you divide the equation for force by sin(phi)

sinmphi_sinphi = sin(iaparams->p.dihedral.mult*phi - 
iaparams->p.dihedral.phase)/sin(phi);

Secondly, when sin(phi) is small how you get this equation

sinmphi_sinphi = iaparams->p.dihedral.mult*
     cos(iaparams->p.dihedral.mult*phi - 
iaparams->p.dihedral.phase)/cosphi;

I would really appreciate your guideline.
Thanks
Best Regards,
Muhammad Anwar